In a manufacturing process of a semiconductor device, a plasma etching process has been performed for forming a pattern. In the plasma etching process of an oxide film, a CF-based gas has been conventionally used as an etching gas.
For the plasma etching process of the oxide film, a method of adding a carbonyl sulfide (COS) gas to the CF-based gas is described in Patent Document 1.
Recently, with the miniaturization of a hole diameter when etching the oxide film to form a hole pattern, it is getting more and more important to control a hole shape after the etching process. Especially, since a bowing is one of prime causes for a decrease in a production yield, the method of adding the COS gas is being attempted in order to ameliorate the problem.    Patent Document 1: Japanese Patent Laid-open Publication No. H09-082688
However, it was found out that sulfur (S) remains on a target substrate after performing the plasma etching process on the oxide film by using the CF-based gas and the COS gas. Since the sulfur (S) may have an adverse effect on human's health if a concentration of the sulfur (S) remaining on the target substrate is high, it would be very important to control the residual sulfur (S) concentration.